1. Field of the Invention
The present invention relates to an optical transmission system for transmitting optical signals using optical fiber.
2. Background Art
Taking into consideration damping and the like of the optical signal in optical communications, it is necessary to increase the optical signal output power in order to increase the distance over which the optical signal can be transmitted. It is known, however, that when the output of the optical signal which is propagating through the fiber and the transmission distance are increased, then the non-linear effect become notable in the optical fiber that is typically employed as the optical transmission path. As a result, this non-linear effect limits the transmission distance which is possible in an optical transmission system.
It is generally known that optical fiber non-linearity typically causes the phenomenon described below.
(1) Self-phase modulation which brings about a phase change in the signal light itself in response to a change in light intensity PA1 (2) Four-wave mixing which causes mutual interaction between signal light of different wavelengths or between signal light and noise light PA1 (3) cross-phase modulation
The phenomenon caused by optical fiber nonlinearity is disclosed in detail in G. P. Agrawal (Ed.), Nonlinear Fiber Optics, Academic Press, for example.
The self-phase modulation effect described above expands the spectrum of the signal light itself, therefore increasing the deterioration in the signal light waveform due to chromatic dispersion in the optical fiber. Chromatic dispersion in the optical fiber which is the cause of this waveform deterioration typically means second order dispersion or higher.
In other words, taking into consideration only the self-phase modulation effect, when desiring to reduce or prevent this effect, it is acceptable to transmit the optical signal at the zero dispersion wavelength of the optical fiber.
On the other hand, four-wave mixing and cross-phase modulation depend on the difference in group velocity between optical signals of differing wavelengths or between the optical signal and light noise. The size of the interaction becomes smaller as the difference in group velocity becomes larger. This difference in group velocity is roughly proportional to the second order dispersion value, so that the second order dispersion value may be made large in order to reduce the four-wave mixing and cross-phase modulation effects. A conventional transmission path for satisfying these reciprocal conditions is arranged as shown in FIG. 7.
FIG. 7 is a diagram showing the arrangement of a conventional transmission path. B1 shows the physical arrangement of the transmission path, B2 shows the distribution of the second order dispersion values of the transmission path shown in B1, and B3 shows the distribution of the third order dispersion values of the transmission path shown in B1.
In the conventional transmission path provided between transmitting device 10 and receiving device 20, unit transmission paths consisting of transmitting fiber 30, optical amplifier 32, transmission fiber 34, optical amplifier 36 and dispersion compensating fiber 38 are connected in a cascade.
Second order dispersion is not zero in transmission fibers 30,34, but rather has a negative dispersion value in the example shown in FIG. 7. In addition, dispersion compensating fiber 38 is for compensating for the dispersion caused when the optical signal is propagated through transmission fibers 30,34, and has a positive second order dispersion value in the example shown in FIG. 7.
Thus, in this way, it has been the conventional practice to employ a combination of optical fibers 30,34 in which the second order dispersion is not zero, and a dispersion compensating fiber 38 which is inserted into each given transmission path so that the second order dispersion value becomes zero.
However, the conventional optical transmission system takes into consideration only the second order dispersion value and is designed to make this second order dispersion value zero. Therefore, the third and higher dispersion possessed by optical fibers which are typically employed does not become zero. In the example shown in FIG. 7, an examination of the third order dispersion value, for example, reveals that transmission fiber 30, transmission fiber 34 and dispersion compensating fiber 38 all have third order dispersion values which are positive, with this dispersion being uncompensated.
For this reason, as a result, the optical signal which has propagated along the transmission path is effected by the third or higher dispersion possessed by the optical fiber. Because conventional optical systems do not take third and higher order dispersion into consideration at all, the entire transmission path is effected.
When the non-linearity of the optical fiber during signal propagation cannot be ignored, a deterioration in the signal waveform occurs, even if a device for compensating for this dispersion is inserted between transmitting device 10 and receiving device 20.
Of the higher order dispersion discussed above, the third order dispersion is particularly problematic as its dispersion value is relatively the largest in comparison with higher order dispersion. Moreover, when the third order dispersion is not zero, then the second order dispersion value differs according to the wavelength.
FIG. 8 is a diagram for explaining the relationship between dispersion characteristics and a signal of multiplexed wavelengths in a conventional optical transmission system.
In FIG. 8, the line denoted by symbol C.sub.p shows the relationship between the second order dispersion value and the wavelength of dispersion compensating fiber 38 in FIG. 7. The line denoted by C.sub.n shows the relationship between the second order dispersion value and the wavelength of the transmission fibers 30,34 in FIG. 7. The line denoted by C.sub.c shows the relationship between the second order dispersion value and the wavelength when transmission fibers 30,34 and dispersion compensating fiber 38 in FIG. 7 are combined.
When carrying out a transmission of multiplexed wavelengths using wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4 as shown in FIG. 8, it is possible to render the second order dispersion value to be zero with respect to the optical signal of a given wavelength (.lambda..sub.2 in the example shown in FIG. 8). However, it is not possible to render the second order dispersion values with respect to the other signal wavelengths (.lambda..sub.1, .lambda..sub.3, and .lambda..sub.4 in the example shown in FIG. 8) to be zero.
As a result, when the signals of these other wavelengths ((.lambda..sub.1, .lambda..sub.3, and .lambda..sub.4 in the example shown in FIG. 8) are propagated along the transmission path, dispersion occurs. Moreover, because this dispersion on the transmission path is not compensated, second order dispersion accumulates over the entire system.
When optical fiber non-linearity with respect to the propagating optical signal cannot be ignored, a deterioration in the signal waveform occurs, even if dispersion compensation is performed at transmitting device 10 or receiving device 20.